The character at the center of Zero Woman: Red Handcuffs is a police officer known only as Rei, often referred to by her call sign, Zero. She is a member of a shadowy and officially non-existent division of the Tokyo police force, a unit designed to handle off-the-books operations and eliminate problems through any means necessary. Her name, Zero, is deeply symbolic, reflecting how the institution views her as a disposable tool and how she, in turn, has come to see herself. When asked about concepts like peace and justice, she dismisses them as meaningless, stating that Zero is the only label that fits.

Rei is defined by a cold, almost psychopathic detachment from the violence and degradation she both endures and inflicts. She operates with a terrifyingly calm demeanor, rarely showing emotion even under extreme duress. This stoicism is her primary defense mechanism, allowing her to compartmentalize her mind from her body when she is tortured, stripped, chained, and subjected to horrific sexual violence by the criminals she is sent to infiltrate. She does not flinch or plead; instead, she waits, watches, and calculates. Her personality is not one of righteous fury but of hollowed-out resolve. She has been so thoroughly brutalized by the system she works for and the underworld she navigates that she exists in a state of emotional numbness, driven purely by mission objectives and a deep-seated, wordless need for a specific kind of lethal justice.

Her background is established in the film's opening sequence. Before becoming the Zero Woman, Rei was a conventional, if excessively zealous, police officer. Her sense of duty, described by a superior as too strong, leads her to assassinate a foreign diplomat who was a serial rapist and murderer and who had diplomatic immunity. This act of extrajudicial killing, however righteous, lands her in prison. She is stripped of her badge and becomes an inmate, discarded by the very institution she served. This incarceration is crucial to her character; it is where she transforms from a rogue cop into the weaponized Zero. Her release is not a pardon but a transaction. A powerful politician, Nagumo, needs a deniable asset to rescue his kidnapped daughter from a gang of psychopaths and eliminate all witnesses. The police offer Rei her freedom in exchange for completing this mission with a "no survivors" order. She is a caged animal being unleashed solely because her savagery has become useful again.

Rei's primary motivation is not redemption or a desire to protect the innocent, but a grim acceptance of a transaction that leads to her freedom. However, as the narrative progresses, a more personal motivation emerges. She becomes a living indictment of the corrupt patriarchal system that uses her. The politician Nagumo, her ultimate antagonist, cares nothing for his daughter and eventually orders her death to avoid a scandal, proving he is as monstrous as the criminals. This betrayal of familial bonds seems to resonate with Rei’s own abandonment by the police force. Her mission shifts from a simple extraction to a nihilistic crusade to expose and punish hypocrisy, culminating in her tearing up her police badge in disgust. She seeks not to restore order, but to prove that the people in power are no better than the thugs they want her to kill.

Within the story, Rei serves as the central instrument of violent reckoning, though her role is less of a constant action hero and more of a patient, implacable force. She spends a significant portion of the film enduring abuse at the hands of the gang to earn their trust, using psychological warfare to turn them against one another rather than simply shooting her way out. Her key relationships are purely transactional. With the kidnapped daughter, Kyoko, she forms no bond of sisterhood; she flatly refers to the girl merely as her mission objective. Her relationship with the gang members is one of predator playing at being prey. Her most significant relationship is with the police commander Nagumo, who represents the ultimate betrayal of the state. It is in defying him and his orders that she completes her development, rejecting the handcuffs of the law she once wore willingly.

Rei’s development is minimal in the sense of emotional growth, but significant in terms of ideological awakening. She begins as a loyal soldier who breaks the rules to enforce a personal vision of justice, which lands her in jail. She is then manipulated into being a weapon for a corrupt official. By the film's end, after witnessing the depths of depravity in both criminals and authority figures, she becomes a rogue agent who acts solely on her own terms, destroying the gang and exposing the politician. She does not find peace or humanity; instead, she fully embraces her identity as Zero, a ghost unbound by the laws of a society she has come to despise.

Her notable abilities are as distinctive as her personality. Her signature weapon is a pair of red handcuffs, which she uses not just for restraint but as a brutal bludgeon and strangulation tool. She is also a proficient markswoman, carrying a red pistol that matches her iconic red coat and ring, a color scheme that underscores the blood she spills. Beyond physical prowess, her greatest ability is her endurance. She withstands levels of physical and sexual torture that would break most people, using the pain as a tool to lower her enemies' guards. Her psychological resilience, the ability to detach from her body and emotions, is her superpower. She is a master of manipulation, capable of sowing discord within the gang while appearing to be a broken victim. Ultimately, Rei is defined by her unbreakable will, transforming from a punished prisoner into a grim reaper who holds her own red handcuffs, ready to drag the guilty down with her.